As examples of an optical recording medium (hereinafter referred to as “optical disk”) having an information recording layer formed on a surface of light-incident side, and a transparent resin covering the information recording layer, e.g. optical disks for CD and optical disks for DVD are widely used. Further, in an optical head device for writing and/or reading an information to/from the optical disks for DVD, e.g. a laser diode of 660 nm wavelength band as a light source and an objective lens having a NA (numerical aperture) of from 0.6 to 0.65, are employed.
Heretofore, as optical disks for DVD, an optical disk having a single information recording layer and having a cover thickness (the thickness of cover layer) of 0.6 mm (hereinafter referred to as “single layer DVD optical disk”), and an optical disk (read-only type or readable-writable type) having two information recording layers (hereinafter referred to as “double layer DVD optical disk”) have been developed. In the double layer DVD optical disk, the distance between information recording layers is 55±15 μm and the information recording layers are formed at positions corresponding to cover thicknesses of 0.56 mm and 0.63 mm in the light-incident side.
By the way, in a case of writing and/or reading to/from a double layer DVD optical disk by using an optical head device having an objective lens optimally designed to have minimum aberration for a single layer DVD optical disk having a cover thickness of 0.6 mm, a spherical aberration is generated according to the difference of the cover thickness and convergence of incident light to an information recording layer is deteriorated. In particular, in a case of double-layer DVD optical disk of writing type, deterioration of convergence causes decrease of converging power density, which causes a writing error, such being a problem.
In recent years, in order to improve recording density of an optical disk, an optical disk having a cover thickness of 100 μm (hereinafter referred to as “single layer BD optical disk”) has been developed. An optical head device for writing and/or reading to/from such a single layer BD optical disk, employs e.g. a laser diode of 405 nm wavelength band as a light source and an objective lens having a NA of 0.85. In this case, if the cover thickness varies ±5 μm within the single layer BD optical disk, a large spherical aberration of about 50 mλ is generated in terms of RMS (Root Mean Square) wavefront aberration, which causes a problem that convergence of incident light on the information recording layer is deteriorated.
Further, in a case of double layer optical disk of recording type (hereinafter referred to as “double layer BD optical disk”) having cover thickness of 100 μm and 75 μm, a large spherical aberration generated according to the difference of the cover thickness, causes a writing error, such being a problem.
Heretofore, as means for correcting a spherical aberration caused by the difference of the cover thickness of e.g. such an optical disk, a method of employing a movable lens group or a liquid crystal lens, has been known.
(I) For example, in order to carry out correction of spherical aberration by using a movable lens group, an optical head device 100 shown in FIG. 12 for writing and/or reading to/from an optical disk D, has been proposed (for example, refer to Patent Document 1).
The optical head device 100 comprises a light source 110, an optical system 120 of various types, a photo-acceptance element 130, a control circuit 140 and a modulation/demodulation circuit 150, and further, a first and a second movable lens groups 160 and 170. The first movable lens group 160 includes a concave lens 161, a convex lens 162 and an actuator 163, which exhibits a focal-length variable lens function that a power of the movable lens group 160 is continuously changeable from positive (convex lens) to negative (concave lens) by moving the convex lens 162 fixed to the actuator 163 in an optical axis direction.
By disposing the movable lens group 160 in an optical path to an optical disk D, it becomes possible to adjust a focal point of incident light to an information recording layer of the optical disk D having a different cover thickness, and thus, to correct a spherical aberration containing a power component.
However, in the case of employing the movable lens group 160, there has been a problem that the size of the optical head device 100 becomes larger since the pair of lenses 161 and 162 and the actuator 163 are required, and the mechanical design for the movable lens becomes complicated.
(II) Further, in order to correct a spherical aberration caused by the difference of cover thickness between an optical disk for DVD and an optical disk for CD, an optical head device employing a liquid crystal lens 200 as shown in FIG. 13, has been proposed (for example, refer to Patent Document 2).
The liquid crystal lens 200 has a construction that it comprises a substrate 230 having a flat surface on which a transparent electrode 210 and an alignment film 220 are formed, a substrate 260 having a curved surface symmetric about an axis and having a surface shape S(r) represented by the following formula being a power series of a radius r:S(r)=α1r2+α2r4+α3r6+  (1)                wherein r2=x2+y2                     α1, α2, α3, . . . : constanton which a transparent electrode 240 and an alignment film 250 are formed, and a nematic liquid crystal 270 sandwiched by the substrates 230 and 260.                        
By the way, in the liquid crystal lens 200, when a voltage is applied between the transparent electrodes 210 and 240, alignment of molecules of the liquid crystal 270 changes and the refractive index of the liquid crystal 270 changes. As a result, a transmission wavefront of incident light changes in accordance with refractive index difference between the substrate 260 and the liquid crystal 270.
Here, the refractive index of the substrate 260 equals to the refractive index of the liquid crystal 270 when no voltage is applied. Accordingly, when no voltage is applied to the liquid crystal, transmission wavefront is not changed from that of incident light. On the other hand, when a voltage is applied between the transparent electrodes 210 and 240, a refractive index difference An is generated between the substrate 260 and the liquid crystal 270, and a distribution of optical path difference of transmission light corresponding to Δn×S(r) is generated (refer to Formula (1) for S(r)). Accordingly, it is possible to correct an aberration by fabricating the surface shape S(r) of the substrate 260 so as to correct a spherical aberration caused by the difference of cover thickness of an optical disk D, and by adjusting the refractive index difference An according to applied voltage.
However, in the case of liquid crystal lens 200 described in FIG. 13, since the refractive index change of the liquid crystal 270 in response to applied voltage is at most about 0.3, it is necessary to increase the concave-convex height of S(r) to generate a large optical path difference distribution Δn×S(r) corresponding to a power component for changing a focal point of incident light. As a result, the layer of liquid crystal 270 becomes thicker and a response speed of optical path change in response to applied voltage becomes slower. Particularly, for writing and/or reading to/from a single layer or a double layer DVD optical disk or a BD optical disk, response within 1 second is required to correct a wavefront aberration generated due to variation of cover thickness or switching of recording layer type between a single layer type and a double layer type, which has been a problem.
Further, in the case of liquid crystal lens 200, it is possible to reduce aberration correction amount, namely, optical path difference distribution by using the liquid crystal lens to correct only spherical aberration from which power component is eliminated, whereby it is possible to reduce the thickness of the liquid crystal layer, which is advantageous for fast response speed. However, in a case of fabricating surface shape S(r) of the substrate 260 so as to correct only spherical aberration, when an optical axis of an objective lens for converging incident light on an information recording layer of an optical disk, and an optical axis of the liquid crystal lens are misaligned to each other, a coma aberration is generated. Particularly, at a time of tracking operation in which the objective lens moves about ±0.3 mm in a radial direction of the optical disk, a large aberration due to the misalignment with the liquid crystal lens occurs, and there occurs a problem that convergence on the information recording layer is deteriorated to prevent writing and/or reading.
(III) By the way, in order to develop a substantial lens function capable of changing also a power component corresponding to focal point change of incident light without increasing the thickness of liquid crystal layer, an optical modulation element is also proposed (for example, refer to Patent Document 3). Further, in order to correct spherical aberration generated due to the difference between the cover thicknesses of DVD optical disk and CD optical disk, an optical head device employing an optical modulation element is also proposed. FIG. 14 shows an optical modulation element 300.
The optical modulation element 300 comprises two transparent substrates 310 and 320 substantially in parallel with each other and a liquid crystal 330 sandwiched between them, and a surface of the transparent substrate 310 in the liquid crystal side constitutes a concave-convex portion 340 having a concentric circular blaze shape, and an electrode 350 and an alignment film 360 are formed on each surface in the liquid crystal side of the two transparent substrate 310 and 320. Further, the liquid crystal 330 has an alignment direction substantially in parallel with the transparent substrates at a time of no electric field application, and the alignment direction is substantially perpendicular to the transparent substrates when an electric field is applied.
Here, by configuring any one of the ordinary refractive index no and the extraordinary refractive index ne of the liquid crystal 330 to be substantially equal to the refractive index of the concave-convex portion 340 of the transparent substrate having the blaze shape, the refractive index difference between the liquid crystal 330 and the concave-convex portion 340 changes from Δn(=ne−no) to zero between at a time of no electric field application and at a time of electric field application. For example, by making the refractive index of the concave-convex portion 340 to be no, and making the depth of the concave-convex portion 340 to be Δn×(depth of the concave-convex portion)=(wavelength of light in vacuo), the optical modulation element 300 functions as a Fresnel lens providing a diffraction efficiency of about 100% at a time of no voltage application. On the other hand, when a voltage is applied, the refractive index of the liquid crystal 330 becomes no and the element does not function as a Fresnel lens and transmits entire light. As a result, by switching the presence and absence of applied voltage to the electrodes of the optical modulation element 300, it is possible to switch focal point between two focal points. By employing such an optical modulation element 300 in an optical head device, convergence to information recording layers of optical disks having different cover thicknesses for DVD and CD, is improved. As a result, it is possible to write and/or read optical disks for DVD and CD by employing an objective lens for DVD.
Accordingly, by employing an optical modulation element 300, it is possible to switch the focal point between two positions by switching presence and absence of the applied voltage, but switching of the focal point to a position between them has been difficult. As a result, it has not been possible to reduce a spherical aberration in a case of cover thickness of from 0.56 mm to 0.63 mm required for a single layer and a double layer DVD optical disk, to be an RMS wavefront aberration level of at most 30 mλ. Further, also in a case of a single layer or a double layer BD optical disk, it has not been possible to reduce a spherical aberration generated when the cover thickness varies ±5 μm, to an RMS wavefront aberration level of at most 50 mλ. As a result, it has not been possible to solve a problem that a large spherical aberration generated due to the difference of cover thickness cannot be sufficiently corrected, which causes a writing error.
Patent Document 1: JP-A-2003-115127
Patent Document 2: JP-A-5-205282
Patent Document 3: JP-A-9-230300